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Fluctuations of the Red Blood Cell Membrane: Relation to Mechanical Properties and Lack of ATP Dependence

James Evans ^* , Walter Gratzer ^*, Narla Mohandas , Kim Parker  and John Sleep ^*

^* King's College London, Randall Division of Cell and Molecular Biophysics, Guy's Campus, London, United Kingdom; Red Cell Physiology Laboratory, New York Blood Center, New York, New York; and Department of Bioengineering, Imperial College, London, United Kingdom

Correspondence: Address reprint requests to John Sleep, E-mail: john.sleep{at}kcl.ac.uk.

We have analyzed the fluctuations of the red blood cell membrane in both the temporal (((s^–1)) and spatial (q(m^–1)) frequency domains. The cells were examined over a range of osmolarities leading to cell volumes from 50% to 170% of that in the isotonic state. The fluctuations of the isotonic cell showed an q^–3-dependence, indicative of a motion dominated by bending, with an inferred bending modulus of 9 x 10^–19J. When the cells were osmotically swollen to just below the point of lysis (166% of physiological volume), a q^–1-dependence of the fluctuations supervened, implying that the motion was now dominated by membrane tension; estimated as 1.3 x 10^–4 nm^–1. When, on the other hand, the cells were osmotically dehydrated, the fluctuation amplitude progressively decreased. This was caused by a rise in internal viscosity, as shown by measurements on resealed ghosts containing a reduced hemoglobin concentration, which displayed no such effect. We examined, in addition, cells depleted of ATP, before the onset of echinocytosis, and could observe no change in fluctuation amplitude. We conclude that the membrane fluctuations of the red cell are governed by bending modulus, membrane tension, and cytosolic viscosity, with little or no dependence on the presence or absence of ATP.